A known superconducting motor disclosed in JP2004-23921A (hereinafter referred to as Reference 1) includes a case forming a vacuum heat insulation chamber, an airtight container forming a gas cooling medium chamber arranged in the case, a stator arranged in the airtight container and generating a rotating magnetic field, a refrigerating device supported by the case and cooling the stator, a rotor formed by a superconducting bulk member rotatably arranged in the airtight container relative to the stator, a rotary shaft rotating along with the rotor, and bearings rotatably supporting the rotary shaft to the case. Since the rotor is formed by the superconducting bulk member, it is necessary for the rotor to be cooled to an extremely low temperature state that is equal to or lower than a critical temperature of the superconducting bulk member. Accordingly, the gas cooling medium is filled in the airtight container. Consequently, when the refrigerating device cools the stator within the airtight container, the gas cooling medium within the airtight container is cooled; therefore, the rotor within the airtight container is maintained in the extremely low temperature state due to thermal conduction by use of the gas cooling medium within the airtight container.
In the superconducting motor according to Reference 1, the bearings are arranged in the extremely low temperature state of the case (vacuum heat insulation chamber). Accordingly, a lubricating agent applied to the bearings is also cooled excessively to therefore deteriorate the durability of the bearing. Consequently, a smooth rotating operation of the rotary shaft may be deteriorated.
A known superconducting motor disclosed in JP2006-149007 (hereinafter referred to as Reference 2) includes a case forming a heat insulation chamber, a coolant chamber arranged in the case, a coolant inlet port through which a coolant cooled to an extremely low temperature state is supplied to the coolant chamber, a coolant outlet port through which the coolant is discharged from the coolant chamber, a stator yoke arranged in the coolant chamber and generating a rotating magnetic field, a rotor yoke having a superconducting coil rotatably arranged in the coolant chamber relative to the stator yoke, a rotary shaft rotating along with the rotor yoke, and bearings rotatably supporting the rotary shaft to the case. The coolant is, for example, liquid nitrogen, liquid hydrogen, and the like. Since the rotor yoke has the superconducting coil, it is necessary for the rotor yoke to be cooled to an extremely low temperature state that is equal to or lower than a critical temperature of the superconducting coil. Accordingly, the coolant cooled by a refrigerating device is supplied to the coolant chamber through the coolant inlet port and thereafter cools the stator yoke and the rotor yoke that are arranged in the coolant chamber. Then, the coolant is discharged from the coolant chamber through the coolant outlet port. Consequently, the rotor yoke having the superconducting coil in the coolant chamber is maintained in the extremely low temperature state. In the superconducting motor according to Reference 2, the bearings are arranged at the coolant chamber maintained in the extremely low temperature state so as to face each other. Accordingly, the bearings may be extremely cooled by the coolant. Further, a lubricating agent applied to the bearings may be cooled, therefore excessively increasing the viscosity of the lubricating agent. As a result, the durability of the bearings deteriorates; thereby, a smooth rotating operation of the rotary shaft may be deteriorated.
A need thus exists for a superconducting rotating electrical machine, which is not susceptible to the drawback mentioned above.